The present invention relates generally to an accelerated illuminate response system for controlling a light emitting diode (xe2x80x9cLEDxe2x80x9d), which may be used for monitoring various parameters in an inkjet printing mechanism, for instance, to monitor the type of print media loaded in the printing mechanism, such as paper or transparencies, or to monitor the location of ink droplets on the print media, so the printing mechanism can adjust future printing for optimal images.
Inkjet printing mechanisms use cartridges, often called xe2x80x9cpens,xe2x80x9d which shoot drops of liquid colorant, referred to generally herein as xe2x80x9cink,xe2x80x9d onto a page. Each pen has a printhead formed with very small nozzles through which the ink drops are fired. To print an image, the printhead is propelled back and forth across the page, shooting drops of ink in a desired pattern as it moves. The particular ink ejection mechanism within the printhead may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal printhead technology. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, Hewlett-Packard Company. In a thermal system, a barrier layer containing ink channels and vaporization chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor. By selectively energizing the resistors as the printhead moves across the page, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text).
To clean and protect the printhead, typically a xe2x80x9cservice stationxe2x80x9d mechanism is mounted within the printer chassis so the printhead can be moved over the station for maintenance. For storage, or during non-printing periods, the service stations usually include a capping system which hermetically seals the printhead nozzles from contaminants and drying. Some caps are also designed to facilitate priming by being connected to a pumping unit that draws a vacuum on the printhead. During operation, clogs in the printhead are periodically cleared by firing a number of drops of ink through each of the nozzles in a process known as xe2x80x9cspitting,xe2x80x9d with the waste ink being collected in a xe2x80x9cspittoonxe2x80x9d reservoir portion of the service station. After spitting, uncapping, or occasionally during printing, most service stations have an elastomeric wiper that wipes the printhead surface to remove ink residue, as well as any paper dust or other debris that has collected on the printhead.
To print an image, the printhead is scanned back and forth across a printzone above the sheet, with the pen shooting drops of ink as it moves. By selectively energizing the resistors as the printhead moves across the sheet, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text). The nozzles are typically arranged in linear arrays usually located side-by-side on the printhead, parallel to one another, and perpendicular to the scanning direction, with the length of the nozzle arrays defining a print swath or band. That is, if all the nozzles of one array were continually fired as the printhead made one complete traverse through the printzone, a band or swath of ink would appear on the sheet. The width of this band is known as the xe2x80x9cswath widthxe2x80x9d of the pen, the maximum pattern of ink which can be laid down in a single pass. The media is moved through the printzone, typically one swath width at a time, although some print schemes move the media incrementally by for instance, halves or quarters of a swath width for each printhead pass to obtain a shingled drop placement which enhances the appearance of the final image.
Inkjet printers designed for the home market often have a variety of conflicting design criteria. For example, the home market dictates that an inkjet printer be designed for high volume manufacture and delivery at the lowest possible cost, with better than average print quality along with maximized ease of use. With continuing increases in printer performance, the challenge of maintaining a balance between these conflicting design criteria also increases. For example, printer performance has progressed to the point where designs are being considered that use four separate monochromatic printheads, resulting in a total of over 1200 nozzles that produce ink drops so small that they approximate a mist.
Such high resolution printing requires very tight manufacturing tolerances on these new pens; however, maintaining such tight tolerances is often difficult when also trying to achieve a satisfactory manufacturing yield of the new pens. Indeed, the attributes which enhance pen performance dictate even tighter process controls, which unfortunately result in a lower pen yield as pens are scrapped out because they do not meet these high quality standards. To compensate for high scrap-out rates, the cost of the pens which are ultimately sold is increased. Thus, it would be desirable to find a way to economically control pens with slight deviations without sacrificing print quality, resulting in higher pen yields (a lower scrap-out rate) and lower prices for consumers.
Moreover, the multiple number of pens in these new printer designs, as well as the microscopic size of their ink droplets, has made it unreasonable to expect consumers to perform any type of pen alignment procedure. In the past, earlier printers having larger drop volumes printed a test pattern for the consumer to review and then select the optimal pen alignment pattern. Unfortunately, the small droplets of the new pens are difficult to see, and the fine pitch of the printhead nozzles, that is, the greater number of dots per inch (xe2x80x9cdpixe2x80x9d rating) laid down during printing, further increases the difficulty of this task. From this predicament, where advances in print quality have rendered consumer pen alignment to be a nearly impossible task, evolved the concept of closed-loop inkjet printing.
In closed loop inkjet printing, sensors are used to determine a particular attribute of interest, with the printer then using the sensor signal as an input to adjust the particular attribute. For pen alignment, a sensor may be used to measure the position of ink drops produced from each printhead. The printer then uses this information to adjust the timing of energizing the firing resistors to bring the resulting droplets into alignment. In such a closed loop system, user intervention is no longer required, so ease of use is maximized.
Closed loop inkjet printing may also increase pen yield, by allowing the printer to compensate for deviations between individual pens, which otherwise would have been scrapped out as failing to meet tight quality control standards. Drop volume is a good example of this type of trade-off. In the past, to maintain hue control the specifications for drop volume had relatively tight tolerances. In a closed loop system, the actual color balance may be monitored and then compensated with the printer firing control system. Thus, the design tolerances on the drop volume may be loosened, allowing more pens to pass through quality control which increases pen yield. A higher pen yield benefits consumers by allowing manufacturers to produce higher volumes, which results in lower pen costs for consumers.
In the past, closed loop inkjet printing systems have been too costly for the home printer market, although they have proved feasible on higher end products. For example, in the DesignJet(copyright) 755 inkjet plotter, and the HP Color Copier 210 machine, both produced by the Hewlett-Packard Company of Palo Alto, Calif., the pens have been aligned using an optical sensor. The DesignJet(copyright) 755 plotter used an optical sensor which may be purchased from the Hewlett-Packard Company of Palo Alto, Calif., as part no. C3195-60002, referred to herein as the xe2x80x9cHP ""002xe2x80x9d sensor. The HP Color Copier 210 machine uses an optical sensor which may be purchased from the Hewlett-Packard Company as part no. C5302-60014, referred to herein as the xe2x80x9cHP ""014xe2x80x9d sensor. The HP ""014 sensor is similar in function to the HP ""002 sensor, but the HP ""014 sensor uses an additional green light emitting diode (LED) and a more product-specific packaging to better fit the design of the HP Color Copier 210 machine. Both of these higher end machines have relatively low production volumes, but their higher market costs justify the addition of these relatively expensive sensors.
FIG. 12 is a schematic diagram illustrating the optical construction of the HP ""002 sensor, with the HP ""014 sensor differing from the HP ""002 sensor primarily in signal processing. The HP ""014 sensor uses two green LEDs to boost the signal level, so no additional external amplification is needed. Additionally, a variable DC (direct current) offset is incorporated into the HP ""014 system to compensate for signal drift. The HP ""002 sensor has a blue LED B which generates a blue light B1, and a green LED G which generates a green light G1, whereas the HP ""014 sensor (not shown) uses two green LEDs. The blue light stream B1 and the green light stream G1 impact along location D on print media M, and then reflect off the media M as light rays B2 and G2 through a lens L, which focuses this light as rays B3 and G3 for receipt by a photodetector P.
Upon receiving the focused light B3 and G3, the photodetector P generates a sensor signal S which is supplied to the printer controller C. In response to the photodetector sensor signal S, and positional data S1 received from an encoder E on the printhead carriage or on the media advance roller (not shown), the printer controller C adjusts a firing signal F sent to the printhead resistors adjacent nozzles N, to adjust the ink droplet output. Due to the spectral reflectance of the colored inks, the blue LED B is used to detect the presence of yellow ink on the media M, whereas the green LED G is used to detect the presence of cyan and magenta ink, with either diode being used to detect black ink. Thus, the printer controller C, given the input signal S from the photodetector P, in combination with encoder position signal S1 from the encoder E, can determine whether a dot or group of dots landed at a desired location in a test pattern printed on the media M.
Historically, blue LEDs have been weak illuminators. Indeed, the designers of the DesignJet(copyright) 755 plotter went to great lengths in signal processing strategies to compensate for this frail blue illumination. The HP Color Copier 210 machine designers faced the same problem and decided to forego directly sensing yellow ink, instead using two green LEDs with color mixing for yellow detection. While brighter blue LEDs have been available in the past, they were prohibitively expensive, even for use in the lower volume, high-end products. For example, the blue LED used in the HP ""002 sensor had an intensity of 15 mcd (xe2x80x9cmilli-candlesxe2x80x9d). To increase the sensor signal from this dim blue light source, a 100xc3x97 amplifier was required to boost this signal by 100 times. However, since the amplifier was external to the photodetector portion of the HP ""002 sensor, this amplifier configuration was susceptible to propagated noise. Moreover, the offset imposed by this 100xc3x97 amplifier further complicated the signal processing by requiring that the signal be AC (alternating current) coupled. Additionally, a 10-bit A/D (analog-to-digital) signal converter was needed to obtain adequate resolution with this still relatively low signal.
The HP ""014 sensor used in the HP Color Copier 210 machine includes the same optics as the HP ""002 sensor used in the DesignJet(copyright) 755 plotter, however, the HP ""014 sensor is more compact, tailored for ease in assembly, and is roughly 40% the size of the HP ""002 sensor. Both the HP ""002 and ""014 sensors are non-pulsed DC (direct current) sensors, that is, the LEDs are turned on and remain on through the entire scan of the sensor across the media. Signal samples are spacially triggered by the state changes of the encoder strip, which provides feedback to the printer controller about the carriage position across the scan. At the relatively low carriage speed used for the optical scanning, the time required to sample the data is small compared to the total time between each encoder state change. To prevent overheating the LEDs during a scan, the DC forward current through the LED is limited. Since illumination increases with increasing forward current, this current limitation to prevent overheating constrains the brightness of the LED to a value less than the maximum possible.
The HP ""014 sensor designers avoided the blue LED problem by using a new way to detect yellow ink with green LEDs. Specifically, yellow ink was detected by placing drops of magenta ink on top of a yellow ink bar when performing a pen alignment routine. The magenta ink migrates through yellow ink to the edges of the yellow bar to change spectral reflectance of the yellow bar so the edges of the bar can be detected when illuminated by the green LEDs. Unfortunately, this yellow ink detection scheme has results which are media dependent. That is, the mixing of the two inks (magenta and yellow) is greatly influenced by the surface properties of media. For use in the home printer market, the media may range from a special photo quality glossy paper, down to a brown lunch sack, fabric, or anything in between. While minimum ink migration may occur on the glossy, photo-type media, a high degree of migration will occur through the paper sack or fabric. Thus, ink mixing to determine drop placement becomes quite risky in the home market, because the printer has no way of knowing which type of media has been used during the pen alignment routine.
Another drawback of the HP ""002 sensor and the HP""014 sensor is that they both require printing of an elaborate test pattern on the available media followed by monitoring the pattern with the sensors. This test routine takes about five to seven minutes to perform, a duration which is not suitable for the home market. The LEDs used for both of these sensors are turned on and off with field effect transistor (FET) switches. The LEDs are driven at their nominal maximum DC forward current when initially turned on. During printing of the test pattern, the LEDs are allowed to warm up to their optimum operating temperature and peak luminosity, so the LEDs"" warm up time has no impact upon the duration of the overall test routine. Additionally, the test pattern wastes a sheet of media, which can be relatively expensive in the home market, such as when photographic quality media is used. Thus, it would be desirable to have a monitoring system which monitors an operator""s normal print job and then makes appropriate adjustments to the printing routine.
Thus, it would be desirable to provide an ink drop sensor system that is particularly economical for use in the home printer market so pen alignment and other adjustments may be implemented during printing to provide consumers with fast, easy-to-use, economical inkjet printing mechanisms that produce high quality images.
According to one aspect of the invention, a method is provided for illuminating a light emitting diode, including the step of applying a high current pulse to the light emitting diode for a selected duration to illuminate the diode. After the selected duration, in a driving step, the light emitting diode is driven with a normal drive current. After the selected duration, the method further includes the step of enduring a secondary illuminate response until illumination of the light emitting diode reaches a selected illumination value.
According to another aspect of the invention, an optical sensing system is provided for sensing ink droplets printed on media by an inkjet printing mechanism. The sensing system includes a light emitting diode directed to illuminate selected portions of the media in response to a drive signal. A photodetecting element is directed to receive light reflected from the illuminated selected portions of the media. The photodetecting element generates an output signal that has an amplitude proportional to the reflectance of the media at the illuminated selected portions. The sensing system also includes a driver that generates the drive signal to apply a high current pulse to the light emitting diode for a selected duration, and thereafter, to apply a normal drive current to the light emitting diode during a secondary illuminate response until illumination of the light emitting diode reaches a selected illumination value while sensing the ink droplets.
According to another aspect of the invention, an inkjet printing mechanism may be provided with such an optical sensing system for controlling a light emitting diode to determine information about print media and/or ink droplets printed on media by the printing mechanism.
An overall goal of present invention is to provide an inkjet printing mechanism having such an accelerated illuminate response system for controlling a light emitting diode sensor system.
A further goal of present invention is to provide a method for optically determining the type of print media loaded in the printing mechanism and/or a characteristic of an inkjet droplet printed on media so future drops may be adjusted by the printing mechanism to produce high quality images without user intervention.
Another goal of the present invention is to provide an accelerated illuminate response system for an inkjet printing mechanism that is lightweight, compact and economical, particularly for use in the home or office.